Screen and Input
In my opinion, this is the most important of all of the technical constraints - it should be the first consideration when designing an application. The 1024 x 768 screen resolution cannot be compared to a desktop monitor. At the same time, input must be made through the screen as there isn't an external keyboard or mouse - making further complications. Buttons require additional screen real estate so that they can be pushed with a fingertip.
Storage
The cheaper of the three iPad's has a 16GB's of storage space. This is one of the most important contraints to consider when designing an app' - people who've little space remaining on their device will scrutinise applications which require a lot more closely.
Power
The iPad has a 25 w·h rechargeable lithium-ion polymer battery[1] that lasts 10 hours, like the original iPad. It is charged via USB or included 10 W, 2 A power adapter.Battery power doesn't last indefinately, and without an external power source, users may have limited time using your application.
Processing
The iPad 2 added a dual core Apple A5 processor that Apple claims doubles processing speed and has graphics processing that is up to nine times faster than the previous iPad.Even considering the above, there is a limit to the processing power of the A5 CPU and this should be considered when developing CPU intensive applications.
Connectivity
The iPad supports WiFi and on the 3G model, 3G also - the restraints these technologies incur must be considered. WiFi has limited range and bandwidth. 3G has limited reception, and bandwidth, which is also expensive. App's will generally have limited or no connectivity on the iPad.
Some information from: http://en.wikipedia.org/wiki/IPad_2#Hardware